Objective:

This fundamental research project leverages a long-term study that was established nearly 20 years ago: the USDA Forest Service Long Term Soil Productivity (LTSP) project. Treatments consist of three levels of biomass removal and two levels of compaction in a 3-by-2 factorial completely randomized design with an unharvested control, for a total of seven treatments. The overall objective of this research is to determine the effects of organic matter removal via intensive wood harvest and soil compaction on SOC pools in surface (0–10 cm) and subsurface (20–30 cm) mineral soil two decades after biomass harvest.

Approach:

In this research, surface and subsurface soil from each replicate of the seven treatments will be used to examine long-term SOC dynamics following biomass harvests of varying organic matter removal and soil compaction intensities. No consensus yet exists regarding these effects because identifying changes due to management can be difficult when looking at the entire, heterogeneous, SOC pool. Therefore, to trace C fluxes through the soil matrix, this project separates labile, fast-cycling and slow-cycling SOC pools by using physical and size/density fractionation techniques. The fractionation procedure isolates the mineral-free light fraction (f LF), occluded light fraction (o LF), and mineral-associated or high-density fraction (HF). The f-LF includes plant detritus that is mainly free of mineral particles, has experienced a minimal amount of microbial degradation and is considered unprotected and easily degradable. The o-LF forms through the fragmentation and degradation of f-LF materials and the association of these materials with mineral particles to form aggregates. As organic matter within aggregates continues to decompose, microbially processed C bound to minerals accumulates to form the HF.

Expected Results:

In general, increasing the intensity of biomass removal is expected to result in decreases in the SOC pool size in surface and subsurface soils compared with control treatments. Biomass harvesting affects all of the organic inputs to the soil system, including aboveground debris and belowground organic C sources of fine root turnover, rhizodeposition and mycorrhizae. Additionally, biomass harvesting can influence the flux of new C into surface and subsurface soil, “priming” the soil microbial community, resulting in accelerated decomposition of stabilized old C and reduced SOC mean residence time. Increasing harvest intensity will likely decrease the SOC pool size. It is further hypothesized that harvesting on Spodosol sands will result in the accumulation of organic C compounds in subsurface mineral soil. In Spodosols, the dominant soil-forming process is podsolization, the elluviation of soluble organic compounds from surface layers into deeper soil. Harvesting on Spodosols leads to accelerated podsolization and accumulation of recent organic C compounds in subsurface mineral soil. Therefore, biomass harvesting at the Huron site will likely accelerate the accumulation of C in subsurface soil. It is also expected that soil compaction treatments will increase SOC pools in sandy soils. Sites with higher aboveground productivity will have more belowground C inputs than low productivity sites.

Potential to Further Environmental/Human Health Protection

While intensive biomass harvesting will be beneficial economically through contributions to the U.S. energy portfolio and socially to rural communities, the potential effects on SOC storage are unclear and could either help or hinder environmental goals. Therefore, empirical data is needed to assess whether biomass harvesting causes soil C stocks to be released at higher-than-anticipated flux rates. The manipulative treatments used in the LTSP provide an exciting and unique opportunity to leverage the long-term investment in the ongoing study and experimentally assess long-term effects of aspen biomass harvesting on SOC dynamics.

Supplemental Keywords:

aspen biomass harvesting, carbon storage, soil organic matter

Progress and Final Reports:

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.